The present invention generally relates to television broadcasting systems. More particular, the present invention relates to RF receivers for radio frequency (RF) reception in a variety of tuner systems such as digital and analog TV tuners, video recorders, analog and digital set top boxes, and cable modems.
TV broadcasting systems and broadband cable systems enable consumers to view a large number of TV channels. In North America, program channels are assigned in radio frequency bands from 42 to 890 MHz, each channel has a bandwidth of 6 MHz. Generally, a terrestrial TV channel is spaced at either 6 MHz or 8 MHz in most parts of the world. Some TV channels are also used in cable modem systems for downstream transmission.
In North America, TV channels are grouped into bands. For example, channels 2 through 6 are grouped in VHF-low band (a.k.a band I in Europe), channels 7 through 13 in VHF-high band (band III), and channels 14 through 69 in UHF band (bands IV and V).
Numerous architectures for RF receivers have been published and are in commercial use. The superheterodyne architecture provides high channel selectivity, and hence is the most commonly used architecture for many decades in radio and TV. The superheterodyne uses a double-conversion scheme that has image frequency problems. In order to reject the image frequency in the receiver, surface acoustic wave (SAW) filters are connected to a low noise amplifier (LNA) output to let through the wanted frequency bands and block out their image frequency. The thus image-free RF bands are mixed with a local oscillator to an intermediate frequency (IF). This IF signal is further filtered by a second bandpass filter, which is usually either a SAW filter or a ceramic resonator. This filtered IF signal is finally down-converted to the baseband signal with a second mixer and a second local oscillator (LO) running at the IF frequency.
TV tuners are wideband receivers. Their bands span from 40 MHz to 800 MHz, a frequency variation of 20× from the low VHF band to the high UHF band. In contrast, most cell phone devices are narrowband receivers. For example, the GSM cellular system has a receive frequency band ranging from 925 MHz to 960 MHz, a variation of about 3% from the frequency center. As a consequence, narrowband receivers can use simple mixers (e.g., direct conversion) where the local oscillator is tuned to the same frequency as the desired RF channel; and the local oscillator (LO) frequency can even have square waveforms. However, a square waveform contains harmonics having large magnitude, especially the third and fifth harmonics of the LO frequency.
Direct conversion architectures have been intensively investigated. By eliminating the need of an IF stage, the direct conversion implementations can reduce the component counts associated with the receiver.
However, due to legacy reasons, TV demodulators operate at an intermediate frequency ranging from 30 to 60 MHz instead of at a DC level. For example, the two standard IF frequencies are 36 MHz and 44 MHz. Because of that, a direct conversion tuning device requires a second mixer stage for up-converting the DC channel to an IF output signal. Ideally, the second mixer stage should be able to preserve the image rejection properties achieved by the first mixer without requiring external components.
Image problems can be resolved by two commonly used image rejection architectures: the Hartley architecture and the Weaver architecture. The Hartley architecture has a major drawback: it is sensitive to I-Q mismatches. The image is only partially canceled with gain and phase imbalance. The change of parameters R and C due to process and temperature variation is one of the sources of I-Q mismatch.
FIG. 1 is a simplified schematic diagram of the Hartley architecture. An RF input coupled to an antenna or a cable (not shown) receives an RF signal which is amplified in the low noise amplifier LNA. The amplified RF signal at the output of the LNA is then applied to mixers 110 and 130 and frequency down-converted into an in-phase signal I and a quadrature signal Q, which are further amplified by amplifiers 112 and 132. A DC offset cancelation block DCOC removes any dc components of the amplified I and Q signals. DC-free I and Q signals are then filtered by lowpass filters 114 and 134. Baseband signals 116 and 136 are further amplified by voltage gain control (VGC) amplifiers 118 and 138.
Amplified baseband signals 120 and 140 are applied to a Hartley image-rejection block 170. Hartley image-rejection block 170 includes an RC element 172 and a CR element 182 that together have the effect of shifting signal 120 by 90 degrees in relation with signal 140. Signals 174 and 184 are added in an adder 190 to produce a desired channel 192. Desired channel 192 can further be lowpass filtered by a lowpass filter 194 and is outputted to a baseband processor (not shown) for further processing.
It can be seen that the sum of signals 174 and 184 results in cancellation of the image and leaves only the desired channel. However, the Hartley image-rejection block 170 is sensitive to mismatches. If the gain and phase of the paths for signals 174 and 184 are not perfectly matched, the image is then only partially cancelled. Sources of mismatches include the amplitude and phase error at the outputs (i.e., the sine and cosine waveforms) and the inaccuracy of R and C parameters due to process and temperature variation.
Accordingly, it is the objective of this invention to provide a technical solution to the problems described above, and that this solution can be integrated into a tuner system by using standard CMOS, BiCMOS, or any other integrated circuit processes.